Transmission lines
Outline
Types of transmission lines
parallel conductors
coaxial cables
transmission line wave propagation
Losses
characteristics impedance
incident and reflected wave and
impedance matching
transmission media
Guided
 some form of conductor that provide conduit in
which signals are contained
 the conductor directs the signal
 examples: copper wire, optical fiber
Unguided
 wireless systems – without physical conductor
 signals are radiated through air or vacuum
 direction – depends on which direction the
signal is emitted
 examples: air, free space
transmission media
 Cable transmission media
 guided transmission medium and can be any
physical facility used to propagate EM signals
between two locations
 e.g.: metallic cables (open wire, twisted pair),
optical cables (plastic, glass core)
incident and reflected wave
 Incident voltage
 voltage that propagates from sources toward the load
 Reflected wave
Voltage that propagates from the load toward the
sources
classifications of transmission lines
 Balanced Transmission line
 2 wire balanced line.
 both conductors carry current. But only one
conductor carry signals.
classifications of transmission lines

classifications of transmission lines
 Unbalanced Transmission line
 One wire is at ground potential
 the other wire is at signal potential
 advantages – only one wire for each signal
 disadvantages –reduced immunity to noises
classifications of transmission lines

classifications of transmission lines
 Baluns
 Balanced transmission lines connected to
unbalanced transmission lines
 e.g.: coaxial cable to be connected to antenna
Metallic Transmission Lines types
 Parallel conductors
 Coaxial cable
parallel conductors
 consists of two or more metallic
conductors (copper)
 separated by insulator – air, rubber etc.
 Most common
 Open Wire
 Twin lead
 Twisted Pair (UTP & STP)
parallel conductors
 Open Wire
 two-wire parallel conductors
Closely spaces by air
 Non conductive spaces
 support
 constant distance between conductors (2-6 inches)
 Pro – simple construction
 Contra – no shielding, high radiation loss, crosstalk
 application – standard voice grade telephone
parallel conductors
 Twin lead
 spacers between the two conductor are replaced
with continuous dielectric – uniform spacing
 application – to connect TV to rooftop antennas
 material used for dielectric – Teflon, polyethylene
parallel conductors
 Twisted pair
 formed by twisting two insulated conductors
around each other
 Neighboring pairs is twisted each other to
reduce EMI and RFI from external sources
 reduce crosstalk between cable pairs
parallel conductors
 Unshielded Twisted Pair
 two copper wire encapsulated in PVC
 twisted to reduce crosstalk and interference
 improve the bandwidth significantly
 Used for telephone systems and local area
network
parallel conductors
 UTP – Cable Type
 Level 1 (Category 1)
 ordinary thin cables
 for voice grade telephone and low speed data
 Level 2 (Category 2)
 Better than cat. 1
 For token ring LAN at tx. rate of 4 Mbps
 Category 3
 more stringent requirement than level 1 and 2
 more immunity than crosstalk
 for token ring (16Mbps), 10Base T Ethernet (10Mbps)
parallel conductors
 UTP – Cable Type
 Category 4
 upgrade version of cat. 3
 tighter constraints for attenuation and crosstalk
 up to 100 Mbps
 Category 5
 better attenuation and crosstalk characteristics
 used in modern LAN. Data up to 100Mbps
 Category 5e
 enhanced category 5
 data speed up to 350 Mbps
parallel conductors
 UTP – Cable Type
 Category 6
 data speed up to 550 Mbps
 fabricated with closer tolerances and use more
advance connectors
parallel conductors
 Shielded Twisted Pair (STP)
 wires and dielectric are enclosed in a conductive
metal sleeve called foil or mesh called braid
 the sleeve connected to ground acts as shield –
prevent the signal radiating beyond the
boundaries
parallel conductors
 STP – Category
 Category 5e
Feature individually shielded pairs of twisted wire
 Category 7
 4 pairs
 surrounded by common metallic foil shield and shielded foil
twisted pair
 1Gbps
 Foil twisted pair
Four pairs of 24-AWG copper wires encapsulated in a common
metallic-foil shield with a PVC outer sheath
 to minimize EMI susceptibility while maximizing EMI immunity
 > 1Gbps
 shielded-foil twisted pair
Four pairs of 24-AWG copper wires surrounded by a common
metallic-foil shield encapsulated in a braided metallic shield
 offer superior EMI protection
 > 1Gbps
Coaxial cable
 used for high data transmission
 coaxial – reduce losses and isolate
transmission path
 basics
 center conductor surrounded by insulation
 shielded by foil or braid
Metallic transmission lines
Coaxial cable
Rigid air filled
solid flexible
Guided Media – Coaxial Cable
BNC Connectors
 To connect coaxial cable to devices, it is necessary to use
coaxial connectors.
 The most common type of connector is the Bayone-Neill-Concelman,
or BNC, connectors.
 Types: BNC connector, BNC barrel, BNC T, Type-N, Type-N barrel.
 Applications include cable TV networks, and some traditional
Ethernet LANs like 10Base-2, or 10-Base5.
Two-wire parallel transmission line
electrical equivalent circuit
Characteristic Impedance of a Line
 A terminated transmission line that is matched in its
characteristic impedance is called a matched line
 The characteristic impedance depends upon the electrical
properties of the line, according to the formula:
 The characteristic impedance can be calculated by using Ohm’s
Law:
Zo = Eo / Io
where Eo is source voltage and Io is transmission line current
Z0 
R  jωL
G  jωC
Characteristic Impedance
 The characteristic impedance for any type of transmission line
can be calculated by calculating the inductance and impedance
per unit length
 For a parallel line with an air the dielectric impedance is:
D
Z 0  276 log
r
 Zo = the characteristic impedance (ohms)
 D = the distance between the centers
 r = the radius of the conductor
Coaxial cable
138
D
Z0 
log
d
r
   r 0
c
Z0 = the characteristic impedance (ohms)
D = the diameter of the outer conductor
d = the diameter of the inner conductor
 = the permittivity of the material
r = the relative permittivity or dielectric constant of the medium
0 = the permeability of free space
For extremely high frequencies, characteristic impedance can be given
by
Zo =
L/C
1
 o 0
Wave propagation on Metallic
transmission lines
 Velocity factor
 The ratio of the actual velocity of propagation of EM wave
through a given medium to the velocity of propagation
through vacuum

p
f
V 
V
c
 Vf = velocity factor
 Vp = actual velocity of propagation
 c = velocity of propagation in vacuum
transmission line wave propagation
 rearranged equation V f  c  V p
 the velocity via tx. line depends on the dielectric
constant of insulating material
1
Vp 
r
 ϵr = dielectric constant
 The velocity along tx. line varies with inductance
and capacitance of the cable
transmission line wave propagation
 as
T  LC
 velocity x time = distance
 therefore
D
Vp 
LC
 normalized distance to 1 meter




Vp = velocity of propagation
√LC = seconds
L
= inductance
C = capacitance
distance D
Vp 

time
T
Vp 
1 meters
LC second
transmission line wave propagation
 Question
 A coaxial cable with
 distributed capacitance C = 96.6 pf/H
 Distributed inductance L = 241.56 nH/m
 Relative dielectric constant. ϵr = 2.3
Determine the velocity of propagation and the velocity
factor
Losses



Conductor Losses
 conductor heating loss - I2R
power loss
 the loss varies depends on
the length of the tx. line
Dielectric Heating Losses
 difference of potential
between two conductors of
a metallic tx lines
 Negligible for air dielectric
 increase with frequency for
solid core tx line
Radiation Losses
 the energy of electrostatic
and EM field radiated from
the wire and transfer to the
nearby conductive material
 Reduced by shielding the cable
Losses
 Coupling Losses
 whenever connection is made between two tx line
 discontinuities due to mechanical connection where
dissimilar material meets
 tend to heat up, radiate energy and dissipate power
 Corona
 luminous discharge that occurs between two conductors
of transmission line
 when the difference of potential between lines exceeds
the breakdown voltage of dielectric insulator